The present invention relates to cell structures for resistors and transistors.
Although NPN transistors can be structured in a linear fashion with the emitter inside the base region and the base region in line with the collector region, the same is not true for PNP transistors. Typically, PNP transistors are arranged in a closed structure such as shown in FIG. 1A. A PNP transistor is shown in FIG. 1A surrounded by a P-isolation region 10. The transistor has an emitter 12 surrounded by a base region 14 (with a base contact 19) which is in turn surrounded by two C-shaped collector regions 16. (If only one collector is required, a single, solid region could be used.) A gap 18 between the regions insures that two separate collectors are usable. The gap 18 is kept narrow to prevent conduction between emitter region 12 and isolation region 10 by forcing the current to collector region 16. The need for the closed PNP structure of FIG. 1 with a central emitter means that the PNP transistor requires more room than an NPN transistor and is more difficult to lay out because of the nonrectangular (C) shape of the collectors.
The simpler NPN layout is shown in FIG. 1B with an emitter 20, base 22 and collector 24. As can be seen, this structure takes up less space and enables it to be arranged linearly with other elements, such as a resistor shown in FIG. 1C.
FIG. 1C shows a typical resistor having an elongate P-region 26 with widened regions 28 at each end to give a T-bone structure. The widened regions 28 are necessary to provide enough room for a metal contact 30. The larger contact areas 30 not only make it easier to make a contact, but also enables the resistor values to be matched more carefully. The larger the contact area, the less a small absolute difference in the contact areas between two resistors will make a difference in the resistor value.